1. Field of the Invention
The present invention relates to an apparatus for the registration or measurement of the humidity and acid content of a refrigerant including a capacitive sensing element for placing in contact with the refrigerant, the sensing element having electrodes consisting of two different metals with different normal potentials, and in which apparatus the determination of humidity is carried out by measuring a resistive loss in the sensing element.
2. The Prior Art
In refrigerants, such as freon, CFC, R12, R22, R502, R134a, etc., used in refrigeration and air conditioning circuits, an undesired concentration of not only humidity but also acid may occur or develop. If there is a considerable concentration of humidity, the humidity will directly prevent the refrigeration circuit from functioning, as the valves and the capillary tubes are blocked by ice formed at the low temperatures. If there is a considerable acid content, the acid will have a detrimental effect on the components of the refrigeration circuit, such as compressor, etc.
The increasing awareness of the damaging effect of some refrigerants on the ozone layer in the atmosphere has made purification (recycling) of used refrigerant desirable, and in some countries it has been made compulsary by law. Using this purification it is desired to remove i.a. water and acid in order to raise the refrigerant to the standard applying to new refrigerant.
Consequently, there is a need for effecting a registration, monitoring or measurement of the humidity and acid content of refrigerants in order to prevent the above-mentioned detriment or blocking of the refrigeration circuit or to make sure that the requirements stipulated as to the purity of the refrigerant are complied with when purifying the refrigerant.
It is known to monitor the humidity content of refrigeration circuits by means of a humidity indicator consisting of a housing with a sight glass and some crystals placed under this sight glass. These crystals turn from yellow to green, dependent on the humidity content of the refrigerant. This change of colour can be observed through the sight glass and is an expression of the humidity content. The method is well known and sturdy but very unreliable and quite unsuitable in connection with the purification of refrigerants, as a maximum humidity level of, e.g., 10-15 ppm, cannot be registered at temperatures above 30.degree. C., due to the temperature dependency of the crystals.
For the registration of the acid content of refrigeration circuits a method is known, in which a sample of the compressor liquid is taken and subsequently the pH-value thereof is controlled by a simple litmus test.
A method of testing both the humidity and the acid content by means of "Drager tubes" is also known. This method is chemical and in this case a change of colour occurs as an expression of a change in the concentration of humidity and acid.
The two latter methods cannot be used for continuous monitoring, and besides they are inconvenient and unreliable.
Furthermore, laboratory methods are known which use gas chromatography and mass spectrometry, but these methods are self-evidently very cost demanding and irrelevant in the present connection.
Apparatuses of the type mentioned above, but which are merely designed for measuring the humidity content of liquids, air or solid materials, such as grain, are disclosed in e.g. GB patent applications Nos. 2,149,117 A, and 2,045,442 A and in DK published patent application No. 129,603. The impedance of the capacitive sensing element varies, dependent on the humidity content of the material with which the sensing element is in contact, and is measured or registered by the electrical circuit which may assume different forms to deliver an electric output signal or to effect a regulation, dependent on the measured humidity.
SE patent No. 370,801 discloses an apparatus for measuring humidity wherein two electodes of different materials with different normal potentials are fed by a D.C. voltage source and are connected to a detector for sensing the potential difference between the electrodes. The indicator is in the form of a differential amplifier, which, e.g., may be coupled in such a manner that different D.C. voltage levels are delivered on its output, dependent on whether there is humidity between the electrodes or not, or whether the electrodes are short-circuited. This prior art apparatus is designed merely to indicate the humidity condition between the electrodes.